1. Field of the Invention
The present invention relates to a metal vapor discharge lamp and, more particularly, to a metal vapor discharge lamp having a built-in starting circuit.
2. Description of the Prior Art
Metal vapor discharge lamps such as a high-voltage sodium lamp are widely used in a field of lighting for sports, commercial facilities, etc. by virtue of the high luminous efficiency and relatively good color rendering. Since it is difficult to start a high-voltage sodium lamp by a commercial supply voltage, for example, a starting circuit having the structure such as that shown in FIG. 1 is used.
A starting device shown in FIG. 1 composed of series circuit having switch 16 and glow starter 18. And the starting circuit is connected in parallel to the light emitting tube 12. Said light emitting tube 12 is connected to an AC power source 10 through a choke coil 14.
A metal vapor discharge lamp having outer bulb (illustrated in dotted line) provided with such a glow starter therein suffers from a problem that when the supply voltage is 100 to 130 V, the discharge lamp is instability to start.
This is because although the peak voltage of the pulse is sufficiently high for producing discharge in the light emittimg tube 12, the width of pulse (energy) is so small and the open circuit voltage (hereinunder referred to as "O. C. V.") as boosting voltage is so low as 100 to 130 V that it is difficult to shift to a stable arc discharge. So, it is very difficult to make metal vapor discharge lamp having starting circuit therein for low voltage.